Comminuting apparatus and method

ABSTRACT

An apparatus for breaking-up or pulverizing particles of solid material, comprising a pulverizing chamber defined by a wall having substantially a shape of revolution with respect to an axis; at least one rotor disposed coaxially in the interior of said pulverizing chamber and comprising a substantially cylindrical body provided with centrifugal flow means for the particles to be pulverized, said means being uniformly distributed angularly with respect to the axis of said chamber, each flow means extending radially from an internal extraction extremity for said particles to be pulverized, located in an axial receiving zone for said particles, up to an external projection extremity for said particles, located at the outer periphery of said rotor, each said centrifugal flow means having a front surface (with respect to the direction of rotation of said rotor) for displacement of said particles; means for introducing said particles to be pulverized into the axial receiving zone of said rotor; an annular pulverizing target disposed in the interior of said chamber and constituted, if so desired, by the wall of said chamber, surrounding said rotor; evacuation means for said pulverized particles, in communication with the peripheral pulverizing zone in the vicinity of said annular target; a driving shaft for said rotor passing through the wall of said pulverization chamber, said flow means further comprising a blade having a front displacement surface with an entirely concave profile from the internal extraction extremity to the external projection extremity of said particles to be pulverized. The invention also covers a method of utilizing this apparatus comprising the further step of recycling at least part of said pulverized particles by fluidization with a carrier gas in a separation and collection chamber and thence to the pulverization chamber.

United States Patent [191 Roussel et a1. 1

[ Mar. 4, 1975 1 COMMINUTING APPARATUS AND METHOD [75] Inventors: Jacques Roussel, St. Germain D Hauterives; Georges Perrin, Vizille, both of France [73] Assignee: LAir Liquide, Societe Anonyme pour IEtude et lExploitation des procedes Georges Claude, Paris, France 22 Filed: July 23, 1973 21 Appl. No.: 381,594

[30] Foreign Application Priority Data July 27, 1972 France 72.27028 [52] US. Cl 241/5, 241/19, 241/24, 24l/79.1, 241/80, 241/275 [51] Int. Cl. B02c 13/09 [58] Field of Search 241/5, 65, 275, 17, 19, 241/23, 24,65, 66, 79, 79.1, 79.2, 79.3, 80, 97, DIG. 10

[56] References Cited 'UNITED STATES PATENTS 990,633 4/1911 Campbell 241/275 X 1,608,717 11/1926 Bell 241/5 3,065,919 11/1962 Burkett et a1... 241/5 X 3,174,697 3/1965 Bridgewaterm. 241/275 3,180,582 4/1965 Danyluke 241/275 3,545,683 12/1970 Shulte 241/5 3,614,001 10/1971 Beike 241/65 Primary Eraminer-Granville Y. Custer, Jr. Attorney, Agent, or Firm-Y0ung & Thompson [57] ABSTRACT An apparatus for breaking-up or pulverizing particles of solid material, comprising a 'pulverizing chamber defined by a wall having substantially a shape of mm lution with respect to an axis; at least one rotor disposed coaxially in the interior of said pulverizing chamber and comprising a substantially cylindrical body provided with centrifugal flow means for the particles to be pulverized, said means being uniformly distributed angularly with respect to the axis of said chamber, each flow means extending radially from an internal extraction extremity for said particles to be pulverized, located in an axial receiving zone for said particles, up to an external projection extremity for said particles, located at the outer periphery of said rotor, each said centrifugal flow means having a front surface (with respect to the direction of rotation of said rotor) for displacement of said particles; means for introducing said particles to be pulverized into the axial receiving zone of said rotor; an annular pulverizing target disposed in the interior of said chamber and constituted, if so desired, by the wall of said chamber, surrounding said rotor; evacuation means for said pulverized particles, in communication with the peripheral pulverizing zone in the vicinity of said annular target; a driving shaft for said rotor passing through the wall of said pulverization chamber, said flow means further comprising a blade having a front displacement surface with an entirely concave profile from the internal extraction extremity to the external projection extremity of said particles to be pulverized.

The invention also covers a method of utilizing this apparatus comprising the further step of recycling at least part of said pulverized particles by fluidization with a carrier gas in a separation and collection chamber and thence to the pulverization chamber.

19 Claims, 11 Drawing Figures MEMEDHAR 19. 5

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SHEET lUUF 1O COMMINUTING APPARATUS AND METHOD The present invention relates to the grinding of particles of solid material. It is more particularly concerned with a method and apparatus for grinding operating by centrifuging and projection of the particles against a target.

A grinding device of the above type has already been proposed, as for example in French Pat. No. 1,370,993, which comprises the following elements:

1. A relatively-flat grinding chamber, comprising a wall which is cylindrical or has a form of revolution with respect to a vertical axis, and two flat walls, an upper wall and a lower wall.

2. A rotor arranged coaxially inside the grinding chamber, comprising a body and a cover both substantially cylindrical and forming between each other an annular substantially flat space. The cylindrical body is provided with metallic wear bands, angularly displaced in a periodic manner with respect to the axis of the grinding chamber, and clamped between grooves in the said body and the cover. Each wear band is curved with a constant radius and is arranged symmetrically with respect to a radius of the rotor, the concavity of the said band being turned toward the exterior of the rotor.

In consequence, the rotation of the rotor differentiates each wear band into a front portion and a rear portion (according to the direction'of rotation) which are symmetrical with respect to a radius of the rotor and are substantially straight and radial. By putting the rotor in rotation, the front portion of each band plays the part of a centrifugal flow means for the particles to be ground, extending radially from an internal extraction extremity of the said particles, located in an axial zone of the rotor ensuring the reception of the particles to be ground, to an external projection extremity for the said particles, located at the outer periphery of the rotor. In order to ensure satisfactory flow of the particles to be ground, at least the front portion of each wear band has a relatively smooth front surface for the sliding and movement of the particles.

The rear portion of each ban, cooperating with the front portion of the next following wear band (in the direction of rotation of the rotor) plays practically no part in the centrifugal flow of the particles to be ground, but essentially permits the flow of the said particles to be channelled. The rear portion of one band and the front portion of the next following band form in fact a closed gutter for the radial circulation of the particles towards the exterior, having a section which decreases from the interior towards the exterior of the rotor.

In consequence, only the front portions of each wear band, substantially straight and radial, distributed angularly in a uniform manner with respect to the axis of the said chamber, play the part of centrifugal flow means for the particles to be ground.

3. An introduction means for the particles to be ground into the grinding chamber, coupled to the upper wall of this latter, comprising a vertical conduit surmounted by a funnel arranged coaxially with the rotor so as to permit direct communication with the axial reception xone of the said rotor.

4. An indented annular target arranged inside the grinding chamber; this target may be coincident with the wall of the grinding chamber, surrounding the rotor and, facing the external projection extremities of the front portions of the wear bands.

5. A tangential evacuation means for the ground particles, coupled to the lower wall of the grinding chamber, in communication with the peripheral grinding zone in the vicinity of the annular target.

6. A driving shaft for the rotor, passing through the lower wall of the grinding chamber.

In operation, the particles of the material to be ground are introduced vertically into the grinding chamber by means of the vertical introduction conduit, provided with its funnel. In this way, the particles introduced into the axial reception zone of the rotor are brought in by gravity.

By rotation of the rotor, the particles to be ground are then accelerated in a horizontal plane and flow by centrifugal force along each front portion of each wear band, following a substantially straight and radial trajectory, from the internal extraction extremity located in the axial reception zone up to an external projection extremity located at the periphery of the rotor.

The flux of particles to be ground is channelled during the course of centrifuging, in the gutters formed by the co-operation of a front portion and a rear portion belonging to two different wear bands. The radially accelerated particles are then projected at high speed from the external projection extremities towards the annular target. The particles of the material treated are then ground by impact and collision with this target, the ground particles are then removed from the peripheral grinding zone in the vicinity of the annular target, and are evacuated from the grinding chamber through the tangential outlet.

The apparatus and method previously described make it possible to convert the mechanical energy applied to the driving shaft of the rotor to kinetic energy imparted to the particles with good efficiency.

However, the previous proposal did not provide any solution for the problem of the friction between the flow means (front portions of the wear bands) of the rotor and the particles to be ground which exist necessarily during the course of centrifuging the material to be ground, along the front displacement surface of each flow means. It has only been proposed to split-up each flow means into a non-consumable fixed portion (cylindrical body of the rotor) which is practically not subject to any wear, and a consumable removable portion (wear bands) on which is applied the friction of the particles during the course of their centrifuging.

This proposal has therefore not sought to eliminate or substantially diminish the effect of friction but has been satisfied solely by localizing this friction on an easily replaceable part. According; to this proposal, the result is that a non-negligible part of the mechanical energy applied to the driving shaft of the rotor is lost by friction of the particles to be ground on the rotor.

Furthermore, this proposal effectively recognizes that between 10 and 15% of the theoretical kinetic energy of the particles (calculated at the outlet of the rotor) is lost by friction between the wear bands and the particles to be ground. The efficiency of the grinding by impact on the annular target is thus correspondingly diminished.

In addition, the necessity of very frequent replacement of the wear bands and/or changing them over as is proposed, and therefore the necessity of dismantling and re-assembling the grinding device each time, are

not compatible with a continuous exploitation of the grinding apparatus of long duration. This substantially increases the stoppage times of the grinder.

Within the scope of a grinding operation carried out by centrifuging and projection of the particles to be ground against a target, the invention thus proposes a substantial reduction, and in certain cases practically an elimination, of the friction of the particles to be ground against the centrifugal flow means of the acceleration rotor of the material to be ground.

The invention is based on the finding that the considerable friction of the particles to be ground against the centrifugal flow means results from the fact that the profile of the front displacement surface of each flow means moves away considerably from the natural trajectory of centrifuging of the particles to be ground.

By natural centrifuging trajectory, there is understood the trajectory which is followed by the particles to be ground when they are accelerated at the surface of a rotor which does not have any centrifugal flow means, as previously defined, and in consequence at the surface of a rotor in which the displacement surface of the particles does not have any obstacle.

This natural trajectory depends on the characteristics of the rotor (especially on its physical characteristics, such as its surface condition, its geometry, and more particularly its radius), on the physical characteristics of the particles to be ground (in particular on their form, roughness, etc.), on the kinematic characteristics of the particles treated at the moment of their contact with the rotor (especially the distance of the point of contact with respect to the axis of the rotor, the direction, sense and absolute value of their speed). and on the kinematic characteristics of the rotor (especially its speed of rotation).

Generally speaking, it has been found that this trajectory has an essentially concave profile, the running tangent of which, that is to say the tangent at any point of the trajectory, is always to the rear with respect to the direction of rotation of the rotor.

In consequence, the present invention proposes to apply this teaching to the design of appropriate centrifugal flow means of the rotor.

A grinding device according to the invention is characterized in that each centrifugal flow means comprises a blade having a front displacement surface which has an entirely concave profile along the centrifuging trajectory of the particles to be ground, from an internal extraction extremity located in the reception zone for the particles of the rotor to an external projection extre'mity located at the periphery of the rotor, the concavity of this profile being turned in the direction of rotation of the rotor.

Thus, according to the invention, there is impressed on the particles to be ground, accelerated by the rotation of the rotor, a centrifuging trajectory predetermined by the profile chosen for the front displacement surface of each blade. This trajectory is generally very close to the natural. centrifuging trajectory, as previously defined, but it may be coincident with this latter when the profile chosen is strictly identical with the natural trajectory when strictly established by any appropriate means.

Generally speaking, the centrifuging trajectory is therefore entirely concave from the internal extraction extremity up to the external projection extremity. The running tangent of this trajectory is always directed towards the rear with respect to the direction of rotation of the rotor. 1

In consequence, by virtue of the profile chosen according to the invention for the front displacement surface of the centrifugal flow blades, the particles to be ground circulate at the surface of the rotor from one extraction extremity to a projection extremity, with a friction which, if it is not brought to zero, is at least reduced to a minimum value. The result is that the relative initial speed of movement of the particles (determined at the point of contact of these latter with the rotor) is substantially equal in absolute value to the relative final speed of the said particles as determined at the point where the particles leave the rotor.

In a corresponding manner, under the same conditions of reception of the particles on the rotor, and under the same conditions of projection of the particles from the rotor, the kinetic energy imparted to the particles is thus increased at the outlet of the rotor, and in consequence the efficiency of grinding is improved without involving any appreciable wear of the centrifugal flow blades.

Other important characteristic features of the invention and their corresponding advantages will be examined within the framework of the description which follows below, reference being made to the accompanying drawings, in which:

FIG. 1 is a half axial section of a grinding device according to the invention;

FIG. 2 is a view looking on the top with parts broken away, of the grinding device shown in FIG. 1.

FIG. 3 is a plan view of the cylindrical body of a rotor forming part of the grinding machine shown in FIGS. 1 and 2;

FIG. 4 is a diagrammatic plan view ofthe rotor shown in FIG. 3 so as to explain its geometric and kinematic characteristics;

FIG. 5 shows in diagrammatic manner a grinding device incorporating a grinder such as shown in FIGS. 1 to 4;

FIG. 6 shows diagrammatically a grinding apparatus similar to that shown in FIG. 5, incorporated in a fluidization recycling circuit;

FIG. 7 shows diagrammatically a grinding apparatus comprising a recycling circuit such as that shown in FIG. 6;

FIG. 8 shows diagrammatically another grinding apparatus comprising a recycling circuit such as that shown in FIG. 6;

FIG. 9 is a diagramatic view of another grinding apparatus comprising a recycling circuit such as that shown in FIG. 6;

FIG. 10 shows another grinding apparatus in accordance with the invention;

FIG. 11 shows still another grinding apparatus according to the invention.

In all the present description, the same reference numbers will be employed in the figures of the drawings, in order to represent the same structural parts.

In accordance with FIGS. 1 to 4, a grinding device according to the invention, indicated by the general reference number 200 and intended for grinding particles of a solid material, comprises the following elements:

1. A grinding chamber 501 defined by a wall having generally a form of revolution with respect to a vertical axis, and comprising a horizontal upper wall 200a, a lower wall 200b inclined downwards, and also a cylindrical lateral wall 203 gripped between the upper wall 200a and the lower wall 200b by means of screwsand bolts 701;

2. a rotor 201 arranged coaxially in the interior of the grinding chamber 501, comprising a cylindrical body 504 and a cover 503 having an upper axial orifice 505. The cover 503 is held at a distance from the body 504 by screws 700 and forms with this latter a substantially annular space 702.

The cylindrical body 504 shown in FIGS. 3 and 4, is provided with centrifugal flow blades 506 for the particles to be ground, distributed angularly in a uniform manner with respect to the axis 502 of the grinding chamber 501. Each blade 506 extends radially from an internal extraction extremity 506a of the particles to be ground, located in an axial reception zone 507 for the said particles, up to an external projection extremity 506b for the particles to be ground, located at the outer periphery 508 of the rotor.

Each blade 506 also has a front surface 5060 (according to the direction of rotation 509 of the rotor) of displacement of the particles to be ground, and a rear surface 506d for grinding the flow of the material to be ground.

The front sliding surface 5060 has a substantially circular profile and is thus entirely concave along the trajectory of the particles to be ground, from the internal extraction extremity 506a to the external projection extremity 506b. The concavity of this profile is turned in the direction of rotation 509 of the rotor.

In addition, as shown in FIG. 4, at the outer projection extremity 506b, the profile of the front surface 5066 is substantially tangential to the outer circular periphery 508 of the rotor, this profile and this periphery forming between them an angle (1 comprised between 0 and Similarly, at the internal extraction extremity 506a, the profile of the front displacement surface 506C is substantially tangential to the internal circular periphery 510 of the axial reception zone 507, this profile and this periphery forming between them an angle a,, comprised between'O" and 10. As shown in FIGS. 1 to 3, the rear face 506d of a given blade and the front face 5060 of the following blade (according to the direction of rotation 509), the cylindrical body 504 and the cover 503 form a closed flow gutter 511 for the material to be ground.

3. A vertical coaxial conduit 515 for the introduction of the particles to be ground, coupled to the upper wall 20001 of the grinding chamber 501, in communication and vertically in line with the axial reception zone 507 of the rotor.

4. An annular grinding target 203, inclined downwards and provided with indentations 203a, fixed on the lateral wall 203, arranged inside the grinding chamber and surrounding the rotor 201.

5. A tangential evacuation conduit 512, coupled to the lower wall 200b of the grinding chamber 501, communicating by means of the inclination of the lower wall with the peripheral grinding zone adjacent the annular target.

6. A driving shaft 204 for the rotor, passing through the lower wall 200b of the grinding chamber 501. As will be seen later, when the grinding device is intended to work under adjustable pressure, lower than atmospheric pressure, the driving shaft 204 of the rotor 201 co-operates with gaseous guiding and/orsupporting means, such as a labyrinth 220', arranged and coupled to the lower wall 200b of the grinding chamber. The grinding chamber 501 then communicates with vacuum-creating means (not shown in FIG. 1) through the intermediary of the conduit 513, a calibrated gaseous passage being ensured through the labyrinth 220. It will be understood that the grinding device which has just been described forms part of a grinding apparatus comprising many other elements, as shown in FIG.

A feed hopper 250 for the particles to be ground is mounted above the grinder 200 and its grinding chamber. This hopper is provided at its lower extremity with an orifice 229 closable by a gate, ensuring the extraction of the particles to be ground. The hopper 250 communicates with the introduction conduit 515 of the grinder 200 through the intermediary (in the direction of circulation of the particles to be ground) of an intermediate hopper 222 which communicates at its upper extremity with the feed hopper and is provided at its lower extremity with an orifice 224 closable by a gate 260 and ensuring an intermediate extraction of the particles to be ground, and with a rotating distributor 209 arranged between the lower orifice of the intermediate hopper 222' and the introduction conduit 515.

A separation collector or cyclone 210 for the ground particles, which may be mounted inside the grinder, communicates at its upper portion with the grinding chamber of-this latter, and more exactly with the peripheral grinding zone. The cyclone 210 is provided at its lower portion with a closable cone 252 for the recovery and extraction of the ground particles. The lower extremity of the cyclone 210 is in communication by a valve 283 with a recovery vessel 280 for the ground particles, which may furthermore be kept undr an atmosphere by a conduit 281 provided with a valve 282;

The driving shaft of the rotor is driven by a motor 281 and a coupling 221.

The grinding apparatus shown may be intended to work at low temperature, for example so as to make plastic materials brittle for grinding. In this case, the feed hoppers 222 and 250, the grinding chamber or grinder 200, the introduction and! evacuation conduits 515 and 512, the separation collector 210, are arranged in a thermally insulated casing 225.

In this case, a circulation passage for a heat-exchange fluid is arranged in thermal contact at 234 with the separation collector 210, especially with the recovery and extraction cone 252, and at 233 with the evacuation conduit 512 of the grinding chamber. This heatexchange passage communicates at one extremity with an inlet 235 of the heat-exchange fluid (gaseous nitrogen for example) and at the other extremity by a conduit 240 with an injection and distribution device (241, 242, 243) for the cooled fluid, arranged in the feed hooper 250.

In addition, a circulation passage for a refrigerant fluid (liquid nitrogen for example) is arranged in thermal contact at 207 with the introduction conduit 512, at 233 with the intermediate hopper 222, at 226 with the feed hopper 250. This passage communicates at one extremity with an inlet 208 of regrigerant fluid and at the other extremity with an injection and distribution device 228 arranged in the feed hopper 250.

For the reasons indicated below, the grinding apparatus may be intended to work under the adjustable pressure lower than atmospheric pressure. In this case, the conduit 513 with a variable flow-rate, in communication with the labyrinth 220, is coupled to a vacuum pump 211'; a conduit 230 with a variable flow-rate 231 communicates with a vacuum pump 232, and a conduit 273 with a variable flow-rate 274 communicates with the pump 232.

The operation of the apparatus which has just been described is as follows:

By means of the vacuum pumps 211', 232, for reasons which will be explained below, in the intermediate hopper 223, the introduction zone 515, the grinding chamber 501 of the grinder 200, the evacuation zone 512 and the separation collector 210, the pressure is regulated to a value less than atmospheric pressure and preferably comprised between torrs and 200 torrs.

The intermediate hopper 222 is brought to atmospheric pressure, the valve 229 is opened, a given quantity of particles to be ground is introduced, the valve 229 is closed, and by means of the pump 232 the pressure in the intermediate hopper is brought to its regulation pressure, lower than the atmospheric pressure. By means of the rotary distributor 209, the particles to be ground are then introduced progressively into the introduction conduit 515. The particles to be ground are then introduced vertically into the grinding chamber 501 (see FIGS. 1 and 2) in the direction of the arrow fl. The particles thus introduced are then led by gravity into the axial reception zone 507 of the rotor 201, in the direction of the arrow f2.

The rotor 201 .being given a movement of rotation (direction 509), the particles thus brought in are then accelerated horizontally in the direction of the arrowf3 during their flow by centrifuging (see FIGS. 3 and 4) from an internal extraction extremity 506a to an external projection extremity 506b.

By virtue of the profile of the front displacement faces 5060 defined above, there is impressed on the particles to be ground, during the course of acceleration, a centrifuging trajectory which is pre-determined by the front shape of the blades 506 which are entirely concave from the internal extremity 506a to the external extremity 506b, and the concavity of which is turned in the direction 509 of rotation of the rotor.

The particles thus accelerated are projected from an external extremity 506b towards the annular target 203 and its indented surfaces 203a, and in this way the particles projected by impact on the target 203 are ground.

Finally, the ground particles are removed from the peripheral grinding zone in the vicinity of the annular target 203, in the direction of the arrows f4 along the inclined wall 200b, and the ground particles are evacuated from the grinding chamber 501 through the tangential evacuation conduit 512.

The ground particles evacuated from the grinder 200 then pass into the separation collector or cyclone 210. The particles are then recovered and extracted in the conical portion of the cyclone 210. In order to evacuate the ground particles, the valve 283 is opened towards an extraction vessel 280, previously put under vacuum. This vessel is filled, the valve 283 is closed, and the vessel 280 is opened to air in order to recover the ground particles.

The essential characteristics and definite advantages of the invention may be readily understood by examining in FIG. 4 the projectory of the particles accelerated on the rotor, and by analyzing their kinematics using the method of the triangle of speeds.

In the first place, it is asumed that the nominal conditions of operation of the grinder have been previously specified as a function of the physical and kinematic characteristics of the rotor and of the particles of the material treated. In other words, it is assumed that the front sliding surface 506a of the blades 506 was determined and shaped especially as a function of the natural centrifuging trajectory of the particles for the speed of rotation chosen for the rotor, and for the absolute speed chosen for the particles coming into contact with the rotor.

In consequence, under the nominal conditions of operation, the particles of the material treated reach an internal extraction extremity 506a with the absolute inlet speed C The speed C is generally horizontal and radial; taking account of the nominal speed of rotation of the rotor, the tangential inlet speed 14, at the internal circular periphery 510 of the axial reception zone 507 determines the existence of a relative input speed w equal to the vector difference between the absolute speed C and the tangential speed u In view of the nominal profile chosen for the front displacement surfaces 5066, the relative input speed w at the internal extraction extremity 506a is tangential to the said front surface.

The particles to be ground are then moved and slide along the front surface 5066. In view of the profile of this surface comes closer or is identical to the natural centrifuging trajectory of the particles, the latter move along the blade 506 without appreciable friction. This means that the relative speed w remains substantially tangential to the centrifuging trajectory and equal in absolute value to its initial value w,, from the extraction extremity 506a to the projection extremity 506b, while the tangential speed it of the rotor increases progressively between these two extremities.

At the external projection extremity 506b, due to the nominal profile chosen for the front face 506c, the relative output speed W2 is substantially tangential to the external periphery 508 of the rotor. The tangential outlet speed 14 and the relative outlet speed W2, the latter being substantially equal in absolute value to the relative inlet speed w are added together as a vector sum so as to give an absolute outlet speed C, at the projection extremity 50612 which is essentially tangential to the external periphery 508 of the rotor.

The particles thus accelerated are then projected at the absolute outlet speed 0 on the impact faces 203a of the annular target 203, at the surface of which the grinding of the particles is effected.

It will be appreciated that the particles are thus evacuated with an absolute maximum speed and therefore with a maximum kinetic energy, due to the kinematic characteristics given below:

a. The relative speed w at the inlet 506a of the rotor is substantially tangential to the front displacement profile 506a. This means that the introduction of the particles on the rotor is effected practically without shock, and therefore without loss of speed;

b. the relative speed w is preserved in absolute value practically from the inlet 506a to the outlet 506b. There is therefore practically no reduction of this speed by friction of the particles along the blades 506;

c. the relative speed W2 at the outlet 50612 of the rotor being practically tangential to the external periphery 508, it is superimposed on the tangential outlet speed n so as to give a maximum absolute outlet speed, practically tangential to the external periphery.

By comparison with the proposal of French Pat. No. 1,370,993, previously discussed, it will be appreciated that for identical conditions of operation, due to the substantially straight and radial profile chosen for the front displacement surface of each centrifugal flow means (front portions of the wear bands):

a. There is a shock at the rotor inlet and therefore a loss of speed;

12. there is friction along the front portions of the wear bands and therefore a further loss of speed of the particles;

0. the relative outlet speed is substantially radial,

which means that by combination with the tangential outlet speed, the absolute outlet speed cannot have a maximum value.

The nature of the particles to be ground may of course vary, whereas the profile of the front displace ment surfaces of the blades remains fixed by construction. In this case, the same trajectory of centrifuging is retained for the particles, and therefore the same kinematic conditions at the inlet and the outlet of the rotor, by regulating at the internal extraction extremities 506a, the absolute speed of reception C of the particles to be ground, and/or the speed of rotation of the rotor, so that the relative reception speed w, of the particles with respect to the rotor is tangential to the centrifuging trajectory pre-determined by the profile of the front face 5060 of the blades 506.

The absolute speed of reception c can be varied in different ways. According to the invention, the operation is preferably carried out in the following manner. With reference to FIG. 5, the pump 21] acts by suction through the labyrinth 220' to produce the passage of a variable flow of gas, circulating with the particles to be ground in the introduction zone 515 towards the grinding chamber 501 of the grinder 200; this flow of gas is finally introduced into the grinding chamber 501.

By regulating this flow-rate of gas, it becomes easy to regulate the absolutespeed of reception 0, of theparticles to be ground. Thus, if the flow of gas corresponds to a linear speed of the gas which is greater than that of the particles, the speed of the particles is increased correspondingly. On the other hand, if this same flowrate of gas corresponds to a linear speed less than that of the particles, the speed of the particles is reduced accordingly.

The pressure of the gaseous regulation flow is chosen as a function of the following conditions. In order to avoid increasing the energy losses of the grinder by stirring up the gas with the rotor of the grinder (fan effect), this pressure must not be too high. On the other hand, in order to obtain an easy regulation of the absolute speed of reception c of the particles on the rotor, the pressure of the gas must not be too low, in order to maintain a viscosity of the gaseous flow compatible with an effective dynamic action of the regulating gas on the particles.

As a result of numerous tests, it has been found according to the invention that the maintenanceof a pressure lower than atmospheric pressure, comprised between 10 torrs and 200 torrs, in the introduction zone 515 of the grinder, constitutes an optimum value for substantially attenuating the fan effect of the grinder while retaining a good dynamic action of the gas on the particles and thus ensuring convenient regulation of the absolute reception speed 0 Furthermore, the regulating flow of gas must not be too large so as not to increase the cost of the corresponding equipment for creating vacuum, and not to increase the fan effect of the rotor. This means that the absolute reception speed c, ofthe particles on the rotor 201 must be as low as possible so as toavoid the necessity of using an excessive flow of gas for accelerating the particles.

Thus, according to the invention, there has been chosen a profile 5066 of the front sliding surface of each blade 506 which is substantially tangential to the internal periphery 510 of the introduction zone 507. The relative speed w, becoming substantially aligned with the tangential speed 14 the absolute reception speed C is thus maintained at its minimum value.

In certain cases, a single passage through the grinder 200 is not sufficient to reduce the particles to be ground to ground particles of pre-determined dimensions. Furthermore, for mechanical reasons, it is often desired to limit the speed of rotation of the grinder below a maximum speed, which involves the existence of a maximum grinding efficiency, and the necessity of proceeding in certain cases to a number of passages of the product to be ground through. the grinder 200. For all these reasons, it may be necessary to proceed to one or more recycling operations of the particles ground in the grinder.

According to FIGS. 6 to 9, the present invention proposes to carry out the recyclingotf the ground particles by fluidization and pneumatic conveyance by means of an appropriate carrier gas.

In accordance with FIG. 6, the recycling circuit proper integrates the grinding chamber 501 of the grinder 200, its introduction conduit 515 and its evacuating conduit 512, the separation. collector or cyclone 210 arranged outside the grinding chamber 501, the feed-hopper 250.

Theseparation collector 210 is arranged on the outside of the grinding chamber, at an average level higher than that of the feed-hopper 250. It communicates at its upper portion by an evacuation conduit 516 with the evacuation means 512 for the ground particles, and at its lower portion with the introduction means 515 of the particles to be ground through the intermediary of a flow conduit 517, of the feed-hopper 250, and of a rotary distributor 520 arranged between this latter and the introduction conduit 515.

The feed-hopper 250, provided at its lower extremity with the rotary distributor 520, communicates at its lower extremity with the introduction means 515 of the grinding chamber 501.

The flow conduit 517 communicates on the one hand with the lower cone 252 of the separation collector 210 and on the other hand with the feed hopper 250. This recycling circuit is associated with circulation means for a carrier gas (for example nitrogen) on the outside of the grinding chamber 501 from this latter towards During recycling, a flow of the carrier gas passes through the calibrated leakage 521 into the introduction zone 515 downstream ofthe rotary valve 521, under a pressure P According to the previous description, this adjustable pressure, maintained in the introduction zone 515, is comprised between 10 torrs and 200 torrs, and the flow of gas introduced through 521 assists in the regulation of the absolute inlet speed of the particles to be ground in the grinder 200.

At the outlet of the grinding chamber 501, in the evacuation conduit 512, the pressure P, is lower than the pressure P,-, but higher than the adjustable pressure P maintained in the collection and separation zone 210. In a similar manner, the ground particles are recyl5 cled by fluidization with the carrier gas in an upward direction inside the separation and collection chamber 210.

By virtue of the leakage flow-rates introduced at 523 and 522, the adjustable pressure P maintained in the 2() feed chamber 250 is lower than the adjustable pressure P, maintained in the collection and separation chamber 210. In a similar way, the ground particles are carried away by fluidization of the collection zone 210 towards the supply zone 250.

Finally, by means of the rotary distributor 520, the introduction zone 515 is caused to communicate with the feed chamber 250, and the ground particles are introduccd into the introduction zone 515 towards the grinding chamber 501.

The operating pressure P P P P and the working pressure P,. of the vacuum pump can be chosen in any otherwise it is impossible to recycle the ground product by fluidization. I

Finally, the working pressures must be distributed as follows:

Similarly, the gaseous flow-rates Qa, Q0 and Qi introduced by the calibrated leakages 522, 523 and 521, and the flow-rate Qv evacuated towards the pump 518 cannot have no matter what value. By a material balance sheet, it is established particularly that these various flow-rates must satisfy the following two equations:

Qa QC Qi Qv Qf+ Qe Qa Or in which:

-Qv is the flow-rate of gas drawn-in by the pump 518;

-Qf is the flow-rate of leakage gas, in counter-flow to the recycling direction, through the rotary valve 520 of the introduction zone 515 under high-pressure towards the supply zone 250 under low pressure;

Qr is the flow-rate of gas carried away by rotation in the direction of recycling by the rotary valve 520 playing the part of a gas circulator;

-Qe is the flow-rate of gas circulating from the collection zone 210 towards the'supply zone 250, through the conduit 517.

The tests carried out below demonstrated the correct operation of the recycling circuit in accordance with the invention.

Speed of Ground rotation of Pa Pi Ps Pc Pv Product grinder in in in in in in r.p.m. torrs torrs torrs torrs torrs Granulated 15,000 67 200 78 -70 -40 polyethylene do. 100 -48 -30 Diamedo. 38 6O 45 -40 -25 ter 3mm: Granulated Bolyethylene l5,000 34 45 44 36.5 -20 iameter E 40 to p.

desired manner in order to ensure correct operation of the recycling circuit.

It has been found surprisingly that the rotary valve 520 continued to operate although the upstream gaseous pressure P (according to the direction of recycling of the particles) is lower than the downstream gaseous pressure P,. However, in order to avoid fluidization of the ground particles in the supply chamber, it is necessary that the difference between the pressures P and P 55 It will be understood that a recycling circuit according to the invention is supplied and evacuated by any appropriate means. Thus, in FIG. 7, the grinding or pul- 50 verizing apparatus shown comprises an evacuation cyclone or extraction means 527 for the ground particles, communicating with an upper outlet 410 of the cyclone 210, that is to say on the downstream side of the grinding chamber 501 of the grinder or pulverizer 200, and on the upstream side of the introduction means 515 of the said chamber, according to the direction of recycling of the ground particles. The feed-hopper 250, which is not incorporated in the recycling circuit, plays the part of a supply means for the particles to be 60 ground, by means of another rotary valve 530 arranged between the said hopper and the introduction means 515 for the particles to be ground.

The feed-hopper 250 being arranged outside the recycling circuit, the rotary valve 520 ensuring the circu- 5 lation of the ground particles from an upstream portion at low pressure to a downstream portion at high pressure, is directly arranged on the extraction conduit 517 at the at the lower portion of the cyclone 210.

The extraction cyclone 527 for the ground particles communicates with a reception vessel 405 for the pulverized product. A motor 525 drives the grinder 200; another hopper 526 for cooling the product to be pulverized communicates with the feed-hopper 250. A cooling circuit, similar to that shown in FIG. 5, ensures the circulation of a refrigerant fluid introduced at 208 into the feed-hopper 250, the introduction conduit 515, the evacuation conduit 516, and the separation collector 210. The refrigerant fluid is finally evacuated into the cooling hopper 526. The putting under vacuum is carried out through filters 270 by vacuum pumps (not shown).

A sorting means for the pulverized particles is incorporated in the collector 210, and comprises the upper outlet 410 for relatively-fine powdered particles, ensuring the extraction of the ground particles to the extraction cyclone 527, and a lower recycling outlet for relatively-large particles, communicating through the rotary valve 520 with the flow conduit 517 connecting together the separation collector 210 and the introduction means 515 of the grinding chamber.

The grinding or pulverizing apparatus shown in FIG. 8 is similar to that shown in FIG. 7, and comprises a recycling circuit incorporating the supply chamber 250 (as shown in FIG. 6). It is also provided with a circulation conduit for a refrigerant fluid introduced at 208, which cools the introduction conduit 515 and the supply hopper 250. The fluid is finally evacuated into the hopper 526. Part of the residual gas is extracted in order to provide the calibrated gas leakage introduced below the rotary valve 520 into the introduction zone 515.

The extraction means or extraction cyclone 527 communicates with the evacuation conduit 516 arranged between the grinding chamber 501 or grinder 200 and the separation collector 210, by means of a change-over device 528 comprising a recycling position 529 to the collector 210, and an evacuation position 530 towards the extraction means 527. i

The grinding apparatus shown in FIG. 9 comprises a recycling circuit identical with that shown in FIG. 6, incorporating the supply or feed-hopper 250. The separation collector 210 is provided with a sorting means 407, the evacuation outlet 410 of which communicates with a receiving vessel 234 for the pulverized product and the recycling outlet 409 of which communicates with the flow conduit 517.

A lock-chamber hopper 222 provides a communication between the feed-hopper 250 and a cooling hopper 526. As previously, the apparatus is provided with a cooling circuit having an inlet 208 and an outlet 228 into the hopper 526, and with a heat-exchange circuit in which the inlet 235 enables the cold to be recovered in the vessel 405, and the outlet which is arranged in the hopper 526.

There is described in FIG. 10 a grinding apparatus according to the invention which makes it possible to obtain progressive pulverization of the particles to be ground. This apparatus comprises a first upper rotor 201a having a relatively-small radius and a last lower rotor 201b having a relatively-large radius, arranged inside the grinding chamber 501, mounted for rotation on the same driving shaft 204.

Each rotor comprises a plurality of flow blades (506a and 506!) respectively) extending radially from an axial receiving zone for the product to be ground (507a and 507b respectively), the dimensions of which are substantially identical for the first and last rotors 201a and 201b, up to the periphery of the corresponding rotor.

The grinding chamber 501 further comprises a means for transfer of the pulverized product by gravity from the peripheral grinding zone 203a associated with the rotor 201a, towards the axial receiving zone 507b of the next following rotor 20lb. The axial receiving zone 507a of the first rotor 201a communicates with the introduction means 515 of the product to be ground by a gutter 104, while the peripheral grinding zone 20312 associated with the last rotor communicates with the evacuation means 512 for the pulverized prod uct.

The grinding chamber 501 communicates with the supply hopper 250 by a lock-chamber provided with valves 15 and 16, while the grinding chamber and the evacuation conduit 512 communicates with each other through a lock-chamber provided with valves 9 and 10. Vacuum-creation means comprise a pump 232 communicating at' 19, 22 and 20 with the two lockchambers and the grinding chamber 501. A conduit 208 for the circulation of a refrigerant fluid enables the grinding and pulverizing apparatus to be cooled.

In FIG. 11 there is described a grinding apparatus in tended to work at low temperatures, without the addition of refrigerant fluid such as liquid nitrogen. This apparatus comprises a grinder 200 identical with that described in FIGS. 1 to 4, and therefore comprising a rotor 201 arranged in a grinding chamber 501, an annular pulverizing target 203, a driving shaft 204 for the rotor, an introduction means 515 for the particles to be ground and an evacuation means 512 for the pulverized particles.

An expansion turbine 550 mounted on the driving shaft 204 of the rotor, is associated with a closed gas circuit for the compression and expansion of a gas, comprising a compressor in which the suction 55lb under low pressure communicates with the outlet 55012 of the expansion turbine 550, and the delivery 5510 of which communicates with the inlet 550a of the turbine.

The passage conduit 552 for the expanded and cold gas,arranged between the suction 55111 of the compressor and the outlet 550b of theturbine, is in heatexchange relation at553 with the annular target 203, and the wall of the grinding chamber 501, and at 554 with the introduction means 515 of the particles to be pulverized.

It should be noted that the expansion turbine 550 and the grinder 200 may be provided as one single component, when the rotor of the turbine 550 and the rotor 201 of the grinding chamber are combined in one unit.

There is thus obtained an apparatus which is perfectly autonomous in refrigerant fluid, the expansion of the gas in closed circuit ensuring on the one hand the drive of the grinder and on the other hand its cooling by exchange with the expanded gas at 553 and 554.

The present invention can be applied to the grinding and pulverization of any material at ambient temperature or at low temperatures.

What we claim is:

1. In an apparatus for grinding particles of solid material comprising:

1. a wall having substantially a shape of revolution about an axis and defining a grinding chamber,

2. at least one rotor disposed coaxially within said grinding chamber, comprising:

a body having a shape of revolution about said axis - centrifugal flow guide means for the particles to be ground, secured to said body, uniformly distributed angularly about said axis, each of said centrifugal flow guide means extending radially from an internal extremity for the extraction of said particles to be ground, located in an axial receiving zone for said particles, up to an external extremity for the projection of said particles, lo-

cated at the outer circular periphery of said rov tor, and each of said centrifugal flow guide means having a front surface with respect to the direction rotation of said rotor for displacement of said particles, said front displacement surface having a concave profile from said internal extraction extremity to said external projection extremity of said particles to be ground,

3. means for introducing said particles to be ground, in communication with said axial receiving zone of said rotor,

4. annular impact means disposed in said grinding chamber, and surrounding said rotor,

5. evacuation means for said ground particles, in communication with a peripheral grinding zone, adjacent to said annular impact means,

6. a driving shaft for said rotor passing through said wall of saidgrinding chamber;

the improvement wherein, in combination:

a. at said external projection extremity, the profile of said front displacement surface of each said centrifugal flow guide means forms an angle comprised between and with said outer circular periphery of said rotor, and

b. at said internal extraction extremity, the profile of said front displacement surface of each said centrifugal flow guide means forms an angle comprised between 0 and 10 with the inner circular periphery of said axial receiving zone.

2. In an apparatus for grinding particles of solid material comprising:

l. a wall having substantially a shape of revolution about an axis and defining a grinding chamber, 2. at least one rotor disposed coaxially within said grinding chamber, comprising: a body having a shape of revolution about said axis centrifugal flow guide means for the particles to be ground, secured to said body, uniformly distributed angularly about said axis, each of said centrifugal flow guide means extending radially from an internal extremity for the extraction of said particles to be ground, located in an axial receiving zone for said particles, up to an external extremity for the projection of said particles, located at the outer circular periphery of said rotor, and each of said centrifugal flow guide means having a front surface with respect to the direction of rotation of said rotor for displacement of said particles, said front displacement surface having a concave profile from said internal extraction extremity to said internal projection extremity of said particles to be ground, 3. means for introducing said particles to be ground, in communication with said axial receiving zone of said rotor,

4. annular impact means disposed in said grinding chamber, and surrounding said rotor,

5. evacuation means for said ground particles, in communication with a peripheral grinding zone, adjacent to said annular impact means, and

6. a driving shaft for said rotor passing through said wall of said grinding chamber;

the improvement wherein said front displacement surface of each said centrifugal flow guide means has a profile, from said internal extraction extremity to said external projection extremity, substantially close to the natural centrifuging trajectory of said particles to be ground under the working conditions of said rotor and said particles.

3. Apparatus according to claim 2, wherein in combination:

a. at said external projection extremity, the profile of said front displacement surface of each said centrifugal flow guide means forms an angle comprised between 0 and 10 with said outer circular periphery of said rotor, and

bat said internal extraction extremity, the profile of said front displacement surface of each said centrifugal flow guide means forms an angle comprised between 0 and 10 with the inner circular periphery of said axial receiving zone.

4. Apparatus according to claim 1, wherein said apparatus works under an adjustable subatmospheric pressure, wherein said driving shaft rotates on gas bearing means comprising a guiding member secured to said wall of said grinding chamber and defining with said driving shaft a calibrated gaseous passage, and wherein vacuum-creating means are connected to said grinding chamber through said calibrated gaseous passage.

5. In an apparatus for grinding particles of solid material comprising:

1. a grinding device comprising:

(1.1) a wall having substantially a shape of revolution about an axis and defining a grinding chamber,

(1.2) at least one rotor disposed coaxially within said grinding chamber, for centrifuging said particles to be ground,

(1.3) means for introducing said particles to be ground into said grinding chamber,

(1.4) annular impact means disposed in said grinding chamber, cooperating with and surrounding said rotor,

(1.5) evacuation means for said ground particles, in communication with a peripheral grinding zone, adjacent to said annular impact means, and

(1.6) a driving shaft for said rotor, passing through said wall of said grinding chamber,

2. supplying means for said particles to be ground, communicating with said introducing means of said grinding device,

3. a rotary distributor for said particles to be ground, disposed between said supplying means and said introducing means of said grinding device, and

4. separating means for said ground particles, communicating with said peripheral grinding zone of said grinding device, and having at least an outlet for said ground particles;

the improvement wherein said apparatus is adapted to work under an adjustable subatmospheric pressure, and so has the further following features:

a. said supplying means comprise a feed hopper under atmospheric pressure, and a depressurizable intermediate hopper, both disposed verti cally,

b. said intermediate hopper has upper and lower closure means, said upper closure means communicating with said feed hopper for the introduction of said particles to be ground, and said lower closure means communicating with said rotary distributor for the extraction of said particles to be ground, and

c. vacuum creating means, to communicate on the one hand with at least said intermediate hopper, and on the other hand through flow-rate adjusting means and at least a filter with at least said grinding chamber, said introducing means, said evacuation means of said grinding device, and said separating means.

6. Apparatus according to claim 5, wherein said separating means have a closable orifice for the evacuation of said ground particles, communicating with a depressurizable recovery vessel for said ground particles.

7. In an apparatus for grinding particles of solid material comprising:

1. a grinding device comprising:

(1.1) a wall having substantially a shape of revolution about an axis and defining a grinding chamber,

(1.2) at least one rotor disposed coaxially within said grinding chamber, for centrifuging said particles to be ground,

(1.3) means for introducing said particles to be ground into said grinding chamber,

(1.4) annular impact means disposed in said grinding chamber, cooperating with and surrounding said rotor,

(1.5) evacuation means for said ground particles, in communication with a peripheral grinding zone, adjacent to said annular impact means, and

(1.6) a driving shaft for said rotor, passing through said wall of said grinding chamber,

2. supplying means for said particles to be ground, disposed above said introducing means of said grinding device, communicating with said introducing means of said grinding device,

3. a rotary distributor for said particles to be ground, disposed between said supplying means and said introducing means of said grinding device,

4. a classifying separator for said ground particles, disposed outside said grinding device, having an inlet for said ground particles, a first outlet for ground particles of larger size, and a second outlet for ground particles of smaller size, and

5. a circuit for the pneumatic recycling of said ground particles of larger size, comprising:

(5.1) an evacuation conduit communicating with said evacuation means of said grinding device, and with inlet of said classifying separator,

(5.2) an introduction conduit communicating with said first outlet of said classifying separator, and with said introducing means of said grinding device, through a rotary distributor,

(5.3) gaseous circulation means for circulating cyclically a carrier gas from said evacuation means of said grinding device to said introducing means of said grinding device, through said evacuation conduit and said classifying; separator, and

(5.4) a carrier gas separator for separating said ground particles of smaller size from said carrier gas, having an inlet communicating with said second outlet of said classifying separator, an outlet for said carrier gas, and an outlet for said ground particles of smaller size;

the improvement wherein said apparatus works under an adjustable subatmospheric pressure, and so has the further following features:

a. said supplying means comprise a feed hopper under atmospheric pressure, and a depressurizable intermediate hopper, both disposed vertically,

b. said intermediate depressurizable hopper has upper and lower closure means, said upper closure means communicating with said feed hopper for the introduction of said particles to be ground, and said lower closure means communicating with said rotary distributor disposed between said supplying means and said introducing means of said grinding device,

0. said gaseous circulating means are vacuumcreating means, communicating on the one hand with said intermediate hopper, and on the other hand through at least one filter with said outlet for said carrier gas of said carrier gas separator, and

(1. said outlet for said ground particles of smaller size of said carrier gas separator has a closable orifice communicating with a recovery vessel for said ground particles of smaller size.

8. In an apparatus for grinding particles of solid material comprising:

' 1. a grinding device comprising:

(1.1) a wall having substantiallya shape of revolution about an axis and defining a grinding chamber,

(1.2) at least one rotor disposed coaxially within said grinding chamber, for centrifuging said particles to be ground,

(1.3) means for introducing said particles to be ground into said grinding chamber,

(1.4) annular impact means disposed in said grinding chamber, cooperating with and surrounding said rotor,

(1.5) evacuation means for said ground particles, in communication with a peripheral grinding zone adjacent to said annular impact means, and

(1.6) a driving shaft for said rotor, passing through said wall of said grinding chamber,

2. supplying means for said particles to be ground, disposed above said introducing means of said grinding device, communicating with said introducing means of said grinding device,

. 3. a rotary distributor for said particles to be ground, disposed between said supplying means and said introducing means of said grinding device,

4. separating means for said ground particles, disposed outside said grinding device, having an inlet communicating with said evacuation means of said grinding device, and an outlet for said ground particles, and

5. means for the pneumatic recycling of said ground particles, comprising:

(5.1) an evacuation conduit communicating with said evacuation means of said grinding device and with said inlet of said separating means, 

1. IN AN APPARATUS FOR GRINDING PARTICLES OF SOLID MATERIAL COMPRISING:
 1. A WALL HAVING SUBSTANTIALLY A SHAPE OF REVOLUTION ABOUT AN AXIS AND DEFINING A GRINDING CHAMBER,
 2. AT LEAST ONE ROTOR DISPOSED COAXILLY WITHIN SAID GRINDING CHAMBER, COMPRISING: - A BODY HAVING A SHAPE OF REVOLUTION ABOUT SAID AXIS - CENTRIFUGAL FLOW GUIDE MEANS FOR THE PARTICLES TO BE GROUND, SECURED TO SAID BODY, UNIFORMLY DISTRIBUTED ANGULARLY ABOUT SAID AXIS, EACH OF SAID CENTRIFUGAL FLOW GUIDE MEANS EXTENDING RADIALLY FROM AN INTERNAL EXTREMITY FOR THE EXTRACTION OF SAID PARTICLES TO BE GROUND, LOCATED IN AN AXIAL RECEIVING ZONE FOR SAID PARTICLES, UP TO AN EXTERNAL EXTREMITY FOR THE PROJECTION OF SAID PARTICLES, LOCATED AT THE OUTER CIRCULAR PERIPHERY OF SAID ROTOR, AND EACH OF SAID CENTRIFUGAL FLOW GUIDE MEANS HAVING A FRONT SURFACE WITH RESPECT TO THE DIRECTION ROTATION OF SAID ROTOR FOR DISPLACEMENT OF SAID PARTICLES, SAID FRONT DISPLACEMENT SURFACE HAVING A CONCAVE PROFILE FROM SAID INTERNAL EXTRACTION EXTREMITY TO SAID EXTERNAL PROJECTION EXTREMITY OF SAID PARTICLES TO BE GROUND,
 2. at least one rotor disposed coaxially within said grinding chamber, comprising: - a body having a shape of revolution about said axis - centrifugal flow guide means for the particles to be ground, secured to said body, uniformly distributed angularly about said axis, each of said centrifugal flow guide means extending radially from an internal extremity for the extraction of said particles to be ground, located in an axial receiving zone for said particles, up to an external extremity for the projection of said particles, located at the outer circular periphery of said rotor, and each of said centrifugal flow guide means having a front surface with respect to the direction rotation of said rotor for displacement of said particles, said front displacement surface having a concave profile from said internal extraction extremity to said external projection extremity of said particles to be ground,
 2. supplying means for said particles to be ground, disposed above said introducing means of said grinding device, communicating with said introducing means of said grinding device,
 2. at least one rotor disposed coaxially within said grinding chamber, for centrifuging said particles to be ground,
 2. supplying means for said particles to be ground, disposed above said introducing means of said grinding device, communicating with said introducing means of said grinding device,
 2. by rotating said rotor, accelerating and centrifuging said introduced particles along a concave trajectory from an internal extraction extremity located in said axial receiving zone up to an external projection extremity located at the outer circular periphery of said rotor,
 2. by rotating said rotor, accelerating and centrifuging said introduced particles along a predetermined concave centrifuging trajectory, from an internal extraction extremity located in said axial receiving zone up to an external projection extremity located at the outer circular periphery of said rotor,
 2. supplying means for said particles to be ground, disposed above said introducing means of said grinding device, communicating with said introducing means of said grinding device,
 2. supplying means for said particles to be ground, communicating with said introducing means of said grinding device,
 2. at least one rotor disposed coaxially within said grinding chamber, comprising: - a body having a shape of revolution about said axis - centrifugal flow guide means for the particles to be ground, secured to said body, uniformly distributed angularly about said axis, each of said centrifugal flow guide means extending radially from an internal extremity for the extraction of said particles to be ground, located in an axial receiving zone for said particles, up to an external extremity for the projection of said particles, located at the outer circular periphery of said rotor, and each of said centrifugal flow guide means having a front surface with respect to the direction of rotation of said rotor for displacement of said particles, said front displacement surface having a concave profile from said internal extraction extremity to said internal projection extremity of said particles to be ground,
 2. In an apparatus for grinding particles of solid material comprising:
 2. a plurality of rotors disposed coaxially within said grinding chamber, for centrifuging respectively and successively said particles to be ground, each of said rotors having an axial reception zone for said particles,
 2. by rotating said rotor, accelerating and centrifuging said introduced particles along a concave trajectory from an internal extraction extremity located in said axial receiving zone up to an external projection extremity located at the outer circular periphery of said rotor,
 3. means for introducing said particles to be ground, in communication with said axial receiving zone of said rotor,
 3. Apparatus according to claim 2, wherein in combination: a. at said external projection extremity, the profile of said front displacement surface of each said centrifugal flow guide means forms an angle comprised between 0* and 10* with said outer circular periphery of said rotor, and b. at said internal extraction extremity, the profile of said front displacement surface of each said centrifugal flow guide means forms an angle comprised between 0* and 10* with the inner circular periphery of said axial receiving zone.
 3. a rotary distributor for said particles to be ground, disposed between said supplying means and said introducing means of said grindIng device, and
 3. projecting the particles so accelerated from said external projection extremity towards said annular impact target means, whereby said projected particles are ground by impact on said impact target means, and
 3. a rotary distributor for said particles to be ground, disposed between said supplying means and said introducing means of said grinding device,
 3. a rotary distributor for said particles to be ground, disposed between said supplying means and said introducing means of said grinding device,
 3. projecting the particles so accelerated from said external projection extremity towards said annular impact target means, whereby said projected particles are ground by impact on said impact target means, and
 3. projecting the particles so accelerated from said external projection extremity towards said annular impact target means, whereby said projected particles are ground by impact on said impact target means, and
 3. means for introducing said particles to be ground, in communication with said axial receiving zone of a first rotor,
 3. means for introducing said particles to be ground into said grinding chamber,
 3. a rotary distributor for said particles to be ground, disposed between said supplying means and said introducing means of said grinding device,
 3. means for introducing said particles to be ground, in communication with said axial receiving zone of said rotor,
 3. MEANS FOR INTRODUCING SAID PARTICLES TO BE GROUND, IN COMMUNICATION WITH SAID AXIAL RECEIVING ZONE OF SAID ROTOR,
 4. ANNULAR IMPACT MEANS DISPOSED IN SAID GRINDING CHAMBER, AND SURROUNDING SAID ROTOR,
 4. annular impact means disposed in said grinding chamber, and surrounding said rotor,
 4. a classifying separator for said ground particles, disposed outside said grinding device, having an inlet for said ground particles, a first outlet for ground particles of larger size, and a second outlet for ground particles of smaller size, and
 4. separating means for said ground particles, disposed outside said grinding device, having an inlet communicating with said evacuation means of said grinding device, and an outlet for said ground particles, and
 4. annular impact means disposed in said grinding chamber, cooperating with and surrounding said rotor,
 4. annular impact means disposed in said grinding chamber, cooperating with and surrounding each of said rotors,
 4. moving away the particles so ground from a peripheral grinding zone, adjacent to said annular impact target means, and evacuating the ground particles from said grinding chamber; the improvement comprising imparting to said particles to be ground, accelerated and centrifuged by rotation of said rotor, a predetermined centrifuging trajectory, from said internal extraction extremity to said external projection extremity, substantially close to the natural centrifuging trajectory of said particles to be ground under the working conditions of said rotor and said particles to be ground.
 4. moving away the particles so ground from a peripheral grinding zone, adjacent to said annular impact target means, and evacuating the ground particles from said grinding chamber; the improvement wherein an adjustable subatmospheric pressure is maintained in said grinding chamber, said adjustable pressure being comprised between 10 and 200 torrs.
 4. a classifying separator for said ground particles, disposed outside said grinding device, having an inlet for said ground particles, a first outlet for ground particles of larger size, and a second outlet for ground particles of smaller size, and
 4. Apparatus according to claim 1, wherein said apparatus works under an adjustable subatmospheric pressure, wherein said driving shaft rotates on gas bearing means comprising a guiding member secured to said wall of said grinding chamber and defining with said driving shaft a calibrated gaseous passage, and wherein vacuum-creating means are connected to said grinding chamber through said calibrated gaseous passage.
 4. separating means for said ground particles, communicating with said peripheral grinding zone of said grinding device, and having at least an outlet for said ground particles; the improvement wherein said apparatus is adapted to work under an adjustable subatmospheric pressure, and so has the further following features: a. said supplying means comprise a feed hopper under atmospheric pressure, and a depressurizable intermediate hopper, both disposed vertically, b. said intermediate hopper has upper and lower closure means, said upper closure means communicating with said feed hopper for the introduction of said particles to be ground, and said lower closure means communicating with said rotary distributor for the extraction of said particles to be ground, and c. vacuum creating means, to communicate on the one hand with at least said intermediate hopper, and on the other hand through flow-rate adjusting means and at least a filter with at least said grinding chamber, said introducing means, said evacuation means of said grinding device, and said separating means.
 4. annular impact means disposed in said grinding chamber, and surrounding said rotor,
 4. moving away the particles so ground from a peripheral grinding zone, adjacent to said annular impact target means, and evacuating the ground particles from said grinding chamber; the improvement wherein for said predetermined centrifuging concave trajectory, and at said internal extraction extremity of said particles to be ground, the relative speed of reception of said particles with respect to said rotor is maintained substantially tangential with respect to said predetermined trajectory by controlling at least one of the following conditions: - the absolute speed of reception of said particles on said rotor - the speed of rotation of said rotor.
 5. evacuation means for said ground particles, in communication with a peripheral grinding zone, adjacent to said annular impact means, and
 5. a circuit for the pneumatic recycling of said ground particles of larger size, comprising: (5.1) an evacuation conduit communicating with said evacuation means of said grinding device, and with inlet of said classifying separator, (5.2) an introduction conduit communicating with said first outlet of said classifying separator, and with said introducing means of said grinding device, through a rotary distributor, (5.3) gaseous circulation means for circulating cyclically a carrier gas from said evacuation means of said grinding device to said introducing means of said grinding device, through said evacuation conduit and said classifying separator, and (5.4) a carrier gas separator for separating said ground particles of smaller size from said carrier gas, having an inlet communicating with said second outlet of said classifying separator, an outlet for said carrier gas, and an outlet for said ground particles of smaller size; the improvement wherein said apparatus works under an adjustable subatmospheric pressure, and so has the further following features: a. said supplying means comprise a feed hopper under atmospheric pressure, and a depressurizable intermediate hopper, both disposed vertically, b. said intermediate depressurizable hopper has upper and lower clOsure means, said upper closure means communicating with said feed hopper for the introduction of said particles to be ground, and said lower closure means communicating with said rotary distributor disposed between said supplying means and said introducing means of said grinding device, c. said gaseous circulating means are vacuum-creating means, communicating on the one hand with said intermediate hopper, and on the other hand through at least one filter with said outlet for said carrier gas of said carrier gas separator, and d. said outlet for said ground particles of smaller size of said carrier gas separator has a closable orifice communicating with a recovery vessel for said ground particles of smaller size.
 5. means for the pneumatic recycling of said ground particles, comprising: (5.1) an evacuation conduit communicating with said evacuation means of said grinding device and with said inlet of said separating means, (5.2) an introduction conduit communicating with said outlet of said separating means, and with said introducing means of said grinding device, through a rotary distributor, (5.3) gaseous circulation means for circulating cyclically a carrier gas from said evacuation means of said grinding device to said introducing means of said grinding device, through said evacuation conduit and said separating means, and (5.4) a carrier gas separator for separating said ground particles from said carrier gas, having an inlet, an outlet for said carrier gas, and an outlet for said ground particles; the improvement wherein: a. said supplying means comprise a feed hopper, and an intermediate hopper, both disposed vertically, said intermediate hopper having a feed inlet communicating with said feed hopper, and an evacuation outlet communicating with said rotary distributor, disposed between said supplying means and said introduction means of said grinding device, b. said introduction conduit communicates with a recycling inlet of said intermediate hopper, whereby said introduction conduit communicates with said introducing means of said grinding device, through said intermediate hopper and said rotary distributor disposed between said supplying means and said introduction means of said grinding device, c. said carrier gas separator is disposed outside the recycling circuit formed by said evacuation means of said grinding device, said evacuation conduit, said separating means, said introduction conduit, said intermediate hopper, said rotary distributor, and said introducing means of said grinding device, and d. change-over means disposed on said introduction conduit, said change-over means haviNg a recycling position from said evacuation means of said grinding device towards said separating means, and an evacuation position from said evacuation means of said grinding device towards said inlet of said carrier gas separator.
 5. In an apparatus for grinding particles of solid material comprising:
 5. evacuation means for said ground particles, in communication with a peripheral grinding zone, adjacent to said annular impact means,
 5. evacuation means for said ground particles, in communication with a peripheral grinding zone adjacent to said annular impact means, and
 5. a circuit for the pneumatic recycling of said ground particles of larger size, comprising: (5.1) an evacuation conduit communicating with said evacuation means of said grinding device, and with said inlet of said classifying separator, (5.2) an introduction conduit communicating with said first outlet of said classifying collector, and with said introducing means of said grinding device, through a rotary distributor, and (5.3) gaseous circulation means for circulating cyclically a carrier gas from said evacuation means of said grinding device to said introducing means of said grinding device, through said evacuation conduit and said classifying separator; the improvement wherein said classifying separator comprises separating means for separating said ground particles from said carrier gas, and classifying means, distinct from said separating means, for separating said ground particles of larger size from said ground particles of smaller size, said separating means having said inlet for said ground particles, an outlet for said carrier gas, and an outlet communicating with an inlet of said classifying means, and said classifying means having said first and second outlets, said second outlet communicting directly with a recovery vessel for said ground particles of smaller size.
 5. evacuation means for said ground particles, in communication with a peripheral grinding zone, adjacent to said annular impact means,
 5. EVACUATION MEANS FOR SAID GROUND PARTICLES, IN COMMUNICATION WITH A PERIPHERAL GRINDING ZONE, ADJACENT TO SAID ANNULAR IMPACT MEANS,
 6. A DRIVING SHAFT FOR SAID ROTOR PASSING THROUGH SAID WALL OF SAID GRINDING CHAMBER; THE IMPROVEMENT WHEREIN, IN COMBINATION: A.AT SAID EXTERNAL PROJECTION EXTREMITY, THE PROFILE OF SAID FRONT DISPLACEMENT SURFACE OF EACH SAID CENTRIFUGAL FLOW GUIDE MEANS FORMS AN ANGLE COMPRISED BETWEEN 0* AND 10* WITH SAID OUTER CIRCULR PERIPHERY OF SAID ROTOR, AND B. AT SAID INTERNAL EXTRACTION EXTERMITY, THE PROFILE OF SAID FRONT DISPLACEMENT SURFACE OF EACH SAID CENTRIFUGAL FLOW GUIDE MEANS FORMS AN ANGLE COMPRISED BETWEEN 0* AND 10* WITH THE INNER CIRCULAR PERIPHERY OF SAID AXIAL RECEIVING ZONE.
 6. a driving shaft for said rotor passing through said wall of said grinding chamber; the improvement wherein said apparatus further comprises:
 6. means for transferring the ground particles from a peripheral grinding zone, adjacent to said annular impact means cooperating with one of said rotors, to said axial receiving zone of the next one of said rotors, and
 6. Apparatus according to claim 5, wherein said separating means have a closable orifice for the evacuation of said ground particles, communicating with a depressurizable recovery vessel for said ground particles.
 6. a driving shaft for said rotor passing through said wall of said grinding chamber; the improvement wherein said front displacement surface of each said centrifugal flow guide means has a profile, from said internal extraction extremity to said external projection extremity, substantially close to the natural centrifuging trajectory of said particles to be ground under the working conditions of said rotor and said particles.
 6. a driving shaft for said rotor passing through said wall of said grinding chamber; the improvement wherein, in combination: a. at said external projection extremity, the profile of said front displacement surface of each said centrifugal flow guide means forms an angle comprised between 0* and 10* with said outer circular periphery of said rotor, and b. at said internal extraction extremity, the profile of said front displacement surface of each said centrifugal flow guide means forms an angle comprised between 0* and 10* with the inner circular periphery of said axial receiving zone.
 7. In an apparatus for grinding particles of solid material comprising:
 7. a single driving shaft passing through the wall of said grinding chamber, on which each of said rotors is secured; the improvement wherein said rotors have respective radii increasing from said introducing means for said particles to be ground to said evacuation means for said ground particles, along the flow path of said particles within said grinding chamber.
 7. at least one expansion turbine mounted on the driving shaft of said rotor, having an inlet for a gas under high pressure, and an outlet for a gas under low pressure, and
 8. a closed circuit for compression and expansion of said gas, comprising: - a compressor having an intake for said gas under said low pressure, and a delivery for said gas under high pressure, said intake and said delivery of said compressor communicating respectivelY with said outlet and said inlet of said expansion turbine, - said expansion turbine, - and at least one of the following elements: - a conduit for said gas expanded to said low pressure, located between said compressor intake and said turbine outlet, in heat exchange relationship with said introducing means for said particles to be ground - a conduit for said gas expanded to said low pressure, located between said compressor intake and said turbine outlet, in heat exchange relationship with said grinding chamber.
 8. In an apparatus for grinding particles of solid material comprising:
 9. Apparatus according to claim 8, wherein said apparatus works under an adjustable subatmospheric pressure, and so has the further following features: a. said intermediate hopper is depressurizable, b. said gaseous circulating means are vacuum-creating means, communicating on the one hand with said depressurizable intermediate hopper, and on the other hand through a filter with said recycling circuit and with said carrier gas separator, and c. said outlet for said ground particles of said carrier gas separator has a closable orifice communicating with a recovery vessel for said ground particles.
 10. In an apparatus for grinding particles of solid material comprising:
 11. Apparatus according to claim 10, and a collection hopper disposed between said supplying means and said rotary distributor disposed between said supplying means and said introduction means of said grinding device, said collection hopper having a feed inlet for said particles to be ground communicating with said supplying means, a recycling inlet for said ground particles of larger size communicating with said introduction conduit, and an outlet for said particles to be grouNd communicating with said rotary distributor, whereby said introduction conduit communicates with said introducing means of said grinding device through said collection hopper and said rotary distributor.
 12. Apparatus according to claim 10, wherein said apparatus works under an adjustable subatmospheric pressure, and so has the further following features: a. said supplying means comprise a feed hopper under atmospheric pressure, and a depressurizable intermediate hopper, both disposed vertically, b. said intermediate depressurizable hopper is provided with closable upper and lower closure means, said upper closure means communicating with said feed hopper for the introduction of said particles to be ground, and said lower closure means communicating with said rotary distributor disposed between said supplying means and said introducing means of said grinding device, c. said gaseous circulating means are vacuum-creating means, communicating on the one hand with said intermediate hopper, and on the other hand through at least one filter and flow-rate adjusting means with said outlet for said carrier gas of said carrier gas separator, and d. said outlet for said ground particles of smaller size of said carrier gas separator has a closable orifice communicating with a recovery vessel for said ground particles of smaller size.
 13. In an apparatus for grinding particles of solid material comprising:
 14. In an apparatus for grinding particles of solid material, adapted to work at low temperature, comprising:
 15. In a method for grinding particles of solid material, utilizing a grinding apparatus comprising a grinding chamber, at least one centrifuging rotor disposed within said grinding chamber, and annular impact target means disposed in said grinding chamber, cooperating with and surrounding said rotor, said method comprising the following steps:
 16. In a method for grinding particles of solid material, utilizing a grinding apparatus comprising a grinding chamber, at least one centrifuging rotor disposed within said grinding chamber, and annular impact target means disposed in said grinding chamber, cooperating with and surrounding said rotor, said method comprising the following steps:
 17. In a method for grinding particles of solid material, utilizing a grinding apparatus comprising a grinding chamber, at least one centrifugiNg rotor disposed within said grinding chamber, and annular impact target means disposed in said grinding chamber, cooperating with and surrounding said rotor, said method comprising the following steps:
 18. A method according to claim 16, wherein: a. said particles to be ground are introduced into said grinding chamber through an introduction zone, b. a carrier gas is also introduced into said introduction zone towards said grinding chamber, said carrier gas circulating in cocurrent flow with respect to said particles to be ground, and c. the flow rate of said carrier gas is adjusted to regulate the absolute speed of reception of said particles to be ground onto said rotor, at said internal extraction extremity.
 19. A method according to claim 18, wherein an adjustable subatmospheric pressure is maintained at least in said introduction zone, and is comprised between 10 and 200 torrs. 